Figure F58. Plot of molar Mg# vs. ppm Zr in gabbroic rocks recovered during Leg 209. Also shown are the compositional field defined by mid-ocean-ridge basalt (MORB) glasses from the Mid-Atlantic Ridge, the average of Mid-Atlantic Ridge (MAR) MORB glass compositions, the composition of "normal MORB" (N-MORB) (from Hofmann, 1988), and average compositions of gabbros from ODP Sites 735B (Natland and Dick, 2002), and 923 (Agar and Lloyd, 1997). MORB glass data were downloaded in April 2003 from PetDB (online at petdb.ldeo.columbia.edu/petdb/). Because (1) Zr is an incompatible element that is almost completely retained in the melt during fractional crystallization of basalt, (2) primitive MORB glasses have ~50 ppm Zr, and (3) average MORB has ~100 ppm Zr, it follows that average MORB probably records ~50% crystal fractionation over the span of liquid Mg#s from ~70% in mantle-derived melts to 60% in average MORB. Gabbroic cumulates in equilibrium with melts in this compositional range should have high Mg#s. For example, using an Fe/Mg clinopyroxene/liquid Kd of 0.23 (e.g., Sisson and Grove, 1993a, 1993b) and assuming that 90% of the iron in melt is ferrous, we estimate that Mid-Atlantic Ridge glasses with Mg#s from 73% to 50% should crystallize clinopyroxene with Mg#s of ~93%82%. Thus, there must be a mass of primitive gabbroic rocks with Mg#s substantially >70% and low Zr concentrations that is approximately equal to the mass of erupted volcanics and sheeted dikes. However, as also noted in the text and in the caption to Figure F57, very few gabbroic rocks sampled from the mid-ocean ridges to date have these characteristics. For example, the average gabbroic rocks from Sites 735 and 923 do not have appropriate compositions to be the complementary cumulates required by MORB fractionation. During Leg 209, only the gabbronorites from Site 1268 and perhaps impregnated peridotites from, for example, Sites 1271 and 1275 could represent part of this complementary refractory cumulate reservoir. Diabases and miarolitic gabbros from Sites 1272 and 1275, together with a few fine-grained gabbroic rocks from Site 1275, have compositions very similar to MORB and are probably chilled liquid compositions. Most of the gabbroic rocks from Site 1275, together with more than one-half of those from Hole 1270B, have such low Mg#s that they cannot play a significant role in the main process of MORB crystal fractionation. Instead, they must crystallize from highly evolved, rarely sampled melts.
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